Energy for interior illumination can be conserved by limiting artificial light to no more than needed for supplementing available daylight to achieve a desired or target level of interior lighting. Since available daylight illumination varies continuously over the course of the day it becomes necessary to adjust the artificial light complement accordingly in order to maintain a constant combined level of illumination. This technique of supplementing available daylight with electric light has come to be known as daylight harvesting. Substantial energy savings can be realized by this method.
Current daylight harvesting technology is limited to use of daylight photosensors external to the electric light fixtures in an effort to minimize introduction of artificial light into the measurement of available ambient daylight. The location of the daylight sensors depends in part upon the type of control being used, whether open loop or closed loop or some combination of these. Open loop systems attempt to isolate the photosensor from the electrical illumination by mounting the sensor outside the interior space being illuminated, such as on a roof or exterior wall of the building, or sometimes on an interior wall but aiming the sensor at an exterior window or skylight. In closed loop systems the photosensor is situated for measuring overall interior illumination and the sensor output is used to adjust the artificial light level to achieve the desired target level of overall interior illumination. Such a closed loop sensor might be installed on a ceiling facing down towards work surfaces such as desktops to measure the total illumination on the work surfaces.
Both types of systems require careful calibration of the photosensor output and the respective control modules to correctly isolate the effect of changes in available daylight upon illumination of the important parts of the interior space. It is also important to compensate for spillover direct illumination of the sensor by the electric interior lights which may distort the measurement of target interior illumination by the sensor.
Wall and ceiling mounted photosensor modules external to the electric light fixtures complicate installation and maintenance of daylight harvesting illumination. A need exists for daylight harvesting lamp fixtures having daylight sensors integral to the light fixture. Such installation presents difficulties which to date have remained unsolved. Mounting the photosensor inside the fixture exposes it to direct illumination by the lamp, saturating the sensor. Conversely, installing the photosensor to one side of the aperture of the light fixture and away from direct exposure to the lamp places the sensor behind ornamental trim which typically surrounds the aperture of the light fixture, and would require an opening in the trim in alignment with the photosensor. This is a significant disadvantage as many interchangeable ornamental trim pieces exist in standard sizes which could not be used without the special opening for the sensor. Even if an opening is provided, the orientation of the trim piece then becomes restricted by the location of the sensor, so that square trim pieces, for example, could not be aligned with walls unless care was taken to install the entire fixture in correct orientation.